The subject matter herein relates generally to radio-frequency (RF) power generators and, more particularly, to mechanisms and methods for tuning a tube amplifier system in an RF power generator.
Radioisotopes (also called radionuclides) have several applications in medical therapy, imaging, and research, as well as other applications that are not medically related. Systems that produce radioisotopes typically include a particle accelerator, such as a cyclotron, that accelerates a beam of charged particles (e.g., H− ions) and directs the beam into a target material to generate the isotopes. The cyclotron includes a particle source that provides the particles to a central region of an acceleration chamber. The cyclotron uses electrical and magnetic fields to accelerate and guide the particles along a predetermined orbit within the acceleration chamber. The magnetic fields are provided by electromagnets and a magnet yoke that surrounds the acceleration chamber. The electrical fields are generated by a pair of radio frequency (RF) electrodes (or dees) that are located within the acceleration chamber. The RF electrodes are electrically coupled to an RF power generator that may include, for example, oscillators, amplifiers, control circuitry, and power supplies. The RF power generator energizes the RF electrodes to provide the electrical field. The electrical and magnetic fields within the acceleration chamber cause the particles to take a spiral-like orbit that has an increasing radius. When the particles reach an outer portion of the orbit, the particles are directed toward the target material for radioisotope production. In addition to controlling the orbit of the particles, the RF electrodes may be used to pull the particles from a particle source in the acceleration chamber.
To operate the RF electrodes within the acceleration chamber, a considerable amount of electric power (e.g., 5 kW to 2 MW) is generated by the RF power generator and delivered to the RF electrodes. The RF power generator includes, among other things, an enclosure that has a tube amplifier system including a power electron vacuum tube. The power electron vacuum tube is hereinafter referred to as a power tube. The power tube may be, for example, a power triode having a cathode, anode, and control grid. The power tube may also be a tetrode or pentode. The tube amplifier system may also include one or more resonators that each have an inner conductor and an outer conductor.
The tube amplifier system may operate at high frequencies, such as within the very high frequency (VHF) range or higher. When operating at such frequencies, each substantial component within the tube amplifier system may have an effect on the ultimate performance of the tube amplifier system. Due to the number of interconnected components and the manufacturing tolerances for each component, it is often necessary to tune the tube amplifier system to achieve a designated performance. For example, it may be necessary to adjust a length of a resonator within the tube amplifier system by moving a shorting deck. For systems that use a pickup loop, moving the shorting deck may render it necessary to re-position the pickup loop. As another example of system tuning, it may be necessary to adjust a capacitor of one of the resonators to provide more or less capacitance.
Adjustments such as those described above can be expensive and/or time-consuming. Moreover, due to the number of components and manufacturing tolerances, the overall tuning process (e.g., number, order, and extent of adjustments) for one system is often different than the tuning processes for other systems. Tube amplifier systems that allow for a more robust or repeatable tuning process are desired.